Direct conversion receiver

Abstract: In a direct conversion receiver in which received radio frequency signals are mixed with quadrature local oscillator signals in I and Q channels, the output signals from the mixers are applied to a phase detector by way of respective frequency tracking filters to reduce the noise bandwidth and improve the sensitivity.

Description: The present invention relates to direct conversion, or homodyne, receivers. Such receivers are commonly used in radio pagers. In European Patent Application EP-A-O 153 835 there is disclosed a direct conversion receiver for a radio pager in which received radio frequency signals are mixed with local oscillator signals in quadrature-phase arms the outputs of which are applied to a detector for recovering data signals represented by the received signals. According to the present invention, a direct conversion radio receiver arrangement in which received radio frequency signals are mixed with local oscillator signals in quadrature-phase arms and output signals from these quadrature-phase arms are applied to a phase detector of the receiver arrangement is characterised in that each of said quadrature-phase arms includes a frequency tracking filter. The frequency tracking filters may comprise, for example, injection locked oscillators, phase locked loops or phase locked filter loops. A direct conversion receiver in accordance with the present invention will now be described with reference to the accompanying drawing, of which: Figure 1 shows schematically a known form of receiver, Figure 2 shows schematically a receiver in accordance with the invention, Figure 3 shows a part of the receiver of Figure 2 in more detail, and Figure 4 shows schematically a variation of part of the receiver shown in Figure 2. Referring first to Figure 1, in the known form of direct conversion receiver, received FSK radio frequency signals are applied to mixers 1 and 2 respectively in I and Q channels or arms, to which mixer signals from a local oscillator 3 are applied in phase quadrature. Output signals from the mixers 1 and 2 are applied to a phase detector 4 by way of respective low pass mixers 5 and 6 and limiters 7 and 8. Referring to Figure 2, in accordance with the present invention, the output signals from the limiters 7 and 8 are applied to the phase detector 4 by way of respective frequency tracking filters 9 and 10, which may take the form of injection locked oscillators, phase locked loops or phase locked filter loops. These frequency tracking filters are arranged to track the difference frequencies generated in the I and Q arms, effectively reducing the noise bandwidth to that required to pass the difference frequency. This improves the signal to noise ratio ahead of the detector 4 and hence improves the sensitivity, by a factor approximating to the square root of the modulation index. Referring now to Figure 3, injection locked oscillators may be preferred as the frequency tracking filters, in view of their simple structure, not requiring any tuning components such as variable capacitance diodes. As shown in Figure 4, the inherent phase shifts experienced in the injection locked oscillators may be used to provide an automatic frequency control signal for the local oscillator 3, by mixing signals Vs obtained from the point shown in Figure 3 with the output signals Vo from the opposite arm in mixers 11 and 12, and integrating the resultant signals to provide an AFC signal. Where simplicity is required one or other of the mixers 11 or 12 may be omitted.

Claims: A direct conversion radio receiver arrangement in which received radio frequency signals are mixed (1, 2) with local oscillator signals (3) in quadrature-phase arms (5, 7 and 6, 8) and output signals from these quadrature-phase arms are applied to a phase detector (4) of the receiver arrangement, characterised in that each of said quadrature-phase arms (5, 7 and 6, 8) includes a frequency tracking filter (9, 10). A direct conversion radio receiver arrangement in accordance with Claim 1 wherein the frequency tracking filters comprise injection locked oscillators. A direct conversion radio receiver arrangement in accordance with Claim 1 wherein the frequency tracking filters (9, 10) comprise phase locked loop circuits. A direct conversion radio receiver arrangement in accordance with Claim 1 wherein the frequency tracking filters (9, 10) comprise phase locked filter loops. A direct conversion radio receiver arrangement in accordance with Claim 3 wherein signals derived from said injection locked oscillators (9, 10) are utilised (11, 12) to derive a signal for controlling the frequency of said local oscillator signals.

Inventors: LAWTON RODNEY JAMES, LAWTON, RODNEY JAMES
Applicants: PLESSEY SEMICONDUCTORS LTD, PLESSEY SEMICONDUCTORS LIMITED
Application year: 1994
Main classification: H03D3
IPC codes: H04L27, H04B1, H03D3